National
Field Manual

5.2.1 INORGANIC CONSTITUENTS

Most filtration systems currently used by the USGS are appropriate for filtering
wholewater samples, if the limitations of each system are taken into account.
Standard USGS procedure is to filter inorganic-constituent
wholewater samples through a 0.45-micrometer
(µm) pore-size disposable capsule filter.
Filtration through media with pore sizes other than 0.45µ m or with
other equipment (such as tangential-flow devices) depends on the use and interpretation
of the data and can yield substantially different results for trace-element
concentrations.

Data-quality requirements for interpretive studies of ground-water and surface-water
chemistry can dictate filtering the sample through a nominal pore size of
0.2 µm. The quality-assurance procedures used for samples filtered through
the 0.45-µm nominal-pore-size capsule, plate, or other filtration equipment
also are required for the
0.2-µm filters. If concentrations of target analytes are analyzed at sub-parts-per-billion
levels, more stringent QA/QC measures are needed. Such samples can be filtered
through a plate filter or other filtration equipment (for example, a 47-mm-diameter
vacuum-filter unit) as long as the equipment used is approved by the study or
program, data-quality requirements are met, and additional quality-control samples
are collected. For additional information on filtration artifacts, procedures,
and equipment, see Kennedy and others (1976), Salonen (1979), McCarthy (1988),
McCarthy and Zachara (1989), Puls and Barcelona (1989), Ward and Harr (1990),
Horowitz and others (1992, 1994), Williams and others (1993), Robards and others
(1994), and Koterba and others (1995).

Cleaning and conditioning of various filter media used for inorganic constituents
are summarized in table 5- 3. Contamination during sample filtration can be
reduced by following the instructions given for cleaning, conditioning, and
handling of the filter media.

Before filtering, designate one member of the processing team
as Clean Hands (CH) and another member as Dirty Hands (DH) if using the
CH/DH method (NFM 4).

Filter the samples within a processing chamber to minimize
the possibility of contamination.

5.2.1.A CAPSULE-FILTER PROCEDURE

The capsule filter is a disposable, self-contained unit composed of a pleated
filter medium encased in a plastic housing that can be connected in-line to
a sample-delivery system (such as a submersible or peristaltic pump) that generates
sufficient pressure (positive or negative) to force water through the filter.
Filter media are available in several other pore sizes, but 0.45 µm is
the pore size used routinely for most studies at this time. The capsule filter
is required for most studies when filtering samples for trace-element analysis
and is recommended when filtering samples for major-ion or other inorganic-constituent
analyses.

The following instructions implement Clean Hands/Dirty Hands (CH/DH) techniques
and the other QA procedures that are required for trace-element samples with
analyte concentrations at the parts-per-billion (ppb) level and that are recommended
as good field practice for all samples.

The DH team member performs operations that are outside of
the processing chamber and the CH team member performs operations inside
the chamber. DH and CH must wear appropriate disposable, powderless gloves
(gloves).

Preclean capsule filters (step 5 below) before
leaving for the field to save field time.

CH: Assemble processing chamber, attach chamber cover,
and change gloves. Place capsule filter and sample bottles into chamber,
and run discharge end of peristaltic pump tubing into the chamber. Open
DIW7 container and cover it with a plastic bag to prevent contamination
from airborne particulates.

3.

CH/DH: (CH) Insert intake end of
peristaltic pump tubing through the plastic covering and into a l-L container
of DIW.

a.

(DH): Attach tubing to peristaltic pump head and pump
DIW to fill tubing.

b.

Discharge waste rinse water through a sink funnel or a toss
(waste) bottle.

4.

Discard DIW stored in DIW-prerinsed sample bottles.
If sample bottles were not DIW-prerinsed by field personnel:

a.

Wearing gloves, rinse off exterior of each bottle.

b.

Pour DIW into bottle until about one-tenth full.

c.

Cap bottle and shake vigorously about five times.

d.

Uncap and empty bottle.

e.

Repeat b-d of step 4 twice (for a total of three times).

f.

Recap bottles until ready to field rinse.

5.

Clean the capsule filter.
If the capsule filter was precleaned, go to the sections that follow
on “To filter a composite sample” or “To filter a pumped sample,” as appropriate.
The steps below comprise sufficient precleaning of the filter for inorganic
analytes at the parts-per-billion (ppb) concentration level. More rigorous
precleaning procedures that include rinsing with trace-metal-grade hydrochloric
acid are required for samples containing ppb concentrations of target analytes
(table 5-3). Only CH touches those portions
of tubing that will be in direct
contact with the DIW or capsule
filter.

Attach pump tubing to inlet connector of capsule filter, keeping
tubing as short as possible. Make sure direction of
flow through capsule filter matches the
direction-of-flow arrow on the side of
the capsule.

•

To help minimize aeration of the sample (usually for ground-water
samples), secure a short length of clean fluorocarbon polymer tubing onto
capsule filter outlet to extend into the sample bottle so the bottle can
be filled from the bottom up.

b.

CH/DH: Pump 1 L of DIW through capsule
filter; discharge waste rinse water through a sink funnel or to a toss bottle.

•

DH operates the pump at a low speed.

•

CH inverts the capsule filter so the arrow on the housing
is pointing up. (This expels trapped air from the capsule during initial
filling; do not allow water to spray onto the chamber walls.)

c.

DH: Remove tubing from DIW reservoir and
continue to operate pump in forward mid-range speed position to drain as
much of the DIW that remains in the capsule filter as possible. While pump
is operating, shake capsule filter to help remove any entrained DIW.

d.

CH: Detach capsule filter from peristaltic
pump tubing, put it into a clean, sealable plastic bag, and place in a corner
of the processing chamber until ready for use.

Filtration procedures differ somewhat, depending on how the sample is collected.
If the sample is collected using discrete collection equipment, such as the
surface-water bag or bottle sampler or ground-water bailer, use the procedures
described below in "To filter a composite sample." If the sample is
collected by pumping it directly from the source, use the procedures described
below in "To filter a pumped sample." Ground-water samples usually
are not collected as a composite. If samples are to be withdrawn from a well
using a bailer, consider using a bailer to which the capsule filter or other
filtration device can be connected inline to the bailer bottom-emptying device.
Pouring a sample from the top of the bailer into another receptacle aerates
the sample and therefore is not a generally recommended procedure for processing
ground-water samples.

To filter a composite sample (generally for surface water):

1.

Field rinse peristaltic pump tubing with the water to be sampled.

a.

CH: Rinse the outside of each end of the peristaltic
pump tubing.

b.

CH: Transfer intake end of peristaltic
pump tubing into composite sample. If a churn splitter is used, transfer
intake end of peristaltic pump tubing through churn funnel and reseal plastic
bag around the tubing.

CH: Discard rinse water through the sink funnel or
into a toss bottle or other receptacle and dispose of appropriately. Prevent
water from ponding in the processing chamber.

e.

DH: Stop peristaltic pump after tubing is field rinsed.

2.

Field rinse capsule filter:

a.

CH: Remove cleaned capsule filter from plastic bag
and attach discharge end of the peristaltic pump tubing to the inlet connector
on the capsule filter.

•

A clean, small plastic hose clamp may be used to secure the
discharge end of the tubing to the capsule filter inlet connector.

•

Check that the direction of sample flow through the capsule
filter matches the direction of the arrow on the capsule.

b.

DH: Operating the pump at low speed, pump sample through
the tubing to the capsule filter.

c.

CH: Turn capsule filter so that the outlet
is pointing up (arrow on capsule housing is pointing up) and flow of the
sample forces trapped air out of the capsule filter while it is filling.
Do not let sample spray onto chamber
cover.

•

The chamber cover must be changed if sample has sprayed onto
it.

•

If some water that sprayed onto the chamber cover has dripped
into the sample bottle, discard the bottle, change the cover, and collect
a new sample.

d.

DH: Stop the peristaltic pump as soon
as the capsule filter is full of sample and all air in the capsule filter
has been expelled.

TECHNICAL NOTE: The goal is to minimize
clogging the filter medium with suspended materials by minimizing the
volume of sample that will be used to field-rinse the filter.

3.

Collect sample filtrate.

a.

CH: Check that there is a tight connection between the pump tubing
and the capsule filter.

DH: Check that the intake tube is properly inserted in the sample
and start the pump.

CH: Collect a maximum of 25 mL of the water to be sampled as it
discharges through the filter. Do not exceed 25
mL.

CH: Field rinse a precleaned 250-mL FA bottle for trace-element
sample only with sample filtrate.

DH: Stop the pump in time to prevent losing filtrate to waste.

CH: Cap bottle, shake vigorously, and then discard rinse water
into appropriate receptacle.

b.

DH: Start pump and resume flow from pump to the filter.

CH: Filter only the next 200 mL
of the sample into the trace-element FA bottle
(fill to top of upper lip of standard 250-mL polyethylene bottle). Cap bottle
securely and set aside for chemical treatment.

c.

DH: Stop the pump after the trace-element FA bottle
is filled.

d.

If a filtered mercury sample is required, restart pump and
repeat steps 3a -c, substituting a FAM bottle for the FA bottle.

e.

CH: Field rinse any remaining sample bottles for inorganic analyses.
Use no more than a total of
100 mL of filtrate per capsule
filter to field rinse any remaining
bottles for filtered sample.

Ground-water samples usually are withdrawn from a well by means of a submersible
pump. Note that this method might be appropriate for some surface-water samples.
The capsule filter or other filter assembly is connected inline with the sample
tubing in order to collect samples directly from the well.

When sampling ground water, DH should check that the turbidity
values recorded at the end of purging have remained stable. Equipment changes
or adjustments that disrupt sample flow can affect sample turbidity and
should be avoided. If sample flow is disrupted, pump for several minutes
until ambient turbidity values are reestablished.

Maintain a smooth, uniform
flow. Do not stop pump or divert flow from capsule filter or
other filter assembly during bottle field rinse or filtration, if possible.

TECHNICAL NOTE: If using a three-way
valve, changing the setting to divert the flow of sample being pumped
to the filter with a submersible pump can cause air bubbles to form, can
air-block the filtration equipment, and can cause changes in pumping rate
that could result in increased turbidity. These effects should be avoided
to preserve sample integrity; therefore, flow to the filter should not
be stopped until all filtration is complete.

1.

Field rinse the capsule filter with sample water:

a.

CH: Ensure that the sample line is full of sample and free of
bubbles; then attach the discharge end of the sample line to the inlet
connector on the capsule filter.

•

Practice your technique for attaching the capsule filter to
the tubing carrying flowing water so that water does not spray onto chamber
walls.

•

Check that the direction of flow matches the direction of
the arrow on the capsule.

b.

DH: Adjust the sample flow through the sample line to the capsule
filter, keeping a slow rate of flow.

c.

CH: Turn the capsule filter so the outlet is pointing
up (arrow on capsule housing is pointing up) and the flow of sample forces
trapped air out while the capsule filter is filling.

•

Do not allow water to spray onto chamber walls.

•

The capsule filter should be full of sample. No air should
be left in the capsule filter.

d.

Field rinse bottles for inorganic-constituent filtered samples with sample
filtrate (section 5.0.3). Use bottles that were already rinsed three times
with DIW. Determine whether the potential clogging of pores in the filter
medium is of concern for your samples (see TECHNICAL NOTE below).

CH: Fill a FA bottle for trace elements with about 200 mL of
sample filtrate (to top of upper lip of 250 mL bottle). Cap bottle and
set aside for chemical treatment.

CH: If a mercury sample is required, field rinse and fill a FAM
bottle using the same procedure as for the 250-mL FA bottle.

CH: Field rinse remaining bottles, trying to use no more than
an additional 100 mL of sample filtrate.

TECHNICAL NOTE: Depending on sample turbidity
and composition, the nominal pore size of filter media tends to decrease
as the volume of sample passed through the filter increases because pores
are clogged by sediment loading or mineral precipitation on the filter (Horowitz
and others, 1994). Ground water with turbidity 5
NTU should not affect filter pore size appreciably. To minimize the chance
of filter clogging, limit the volume of sample passed through the filter
by eliminating the field rinse--be sure that you use clean bottles and fill
them one after the other. For ground-water sampling, do not stop the pump
during the field-rinse and sampling process.

e.

CH: Collect sample filtrate immediately into any remaining bottles
in the following sequence (flow rate should be slow enough to avoid splashing
sample out of the bottle): (1) major cations, (2) major anions and nutrients
(including alkalinity sample for field titration), (3) radiochemicals
(check Appendix A5-A for bottle-rinse and filtration requirements), (4)
stable isotopes.

CH: If equipment will be reused at another site before returning
to the office, rinse immediately with DIW and field clean tubing and other
sample-wetted parts of the equipment using the prescribed cleaning procedures
(NFM 3)

•

CH: If equipment or tubing will not be reused before
returning to the office, rinse immediately with DIW and store rinsed tubing
and equipment in plastic bags for office or laboratory cleaning.

4.

Discard the capsule filter after
filtering each sample--do not reuse.

5.

Document the filtration procedures used on field forms and
in field notes.

Use of the 0.45-µm disposable capsule
filter for trace-element samples is required for many USGS programs.

5.2.1.B PLATE-FILTER PROCEDURE

The filtering procedure using a 142-mm-diameter plastic plate-filter assembly
is described below. The procedure remains basically the same for plate-filter
assemblies of different diameters.

Prepare and precondition plate-filter assembly:

The following instructions pertain to either a 142-mm-diameter or a 47-mm-diameter
plastic plate-filter assembly and require that the assembly components have
been rigorously cleaned (NFM 3). To avoid recleaning in the field, prepare a
set of filtration equipment for each well or surface-water sampling station.
(Ignore Step 3 below if plate-filter assembly has been rinsed in the office.)

1.

CH: Put on gloves. In a processing chamber, open a
clean plate-filter assembly and load with the filter.

a.

Using nonmetallic forceps, place the bottom retaining screen on the base
of the filter assembly. Do not interchange bottom
and top retaining screens.

b.

Place the filter on top of bottom retaining screen using clean,
blunt plastic or ceramic forceps. Do not touch the filter with hands (gloved
or ungloved).

•

Be sure that only one filter is transferred from its original container
directly to the plate of the filter assembly. Take care not to transfer
the paper liner that separates each filter.

•

The filter should never be removed from the original container
until each is transferred to the plate-filter assembly for use. (Exception:
polycarbonate (Nuclepore) filter medium is precleaned with acid solution.
If transferring one of these, hold the filter with forceps and rinse off
acid with inorganic blank water (IBW) dispensed from wash bottle.)

c.

Using forceps, place the top retaining screen on top of the
filter.

TECHNICAL NOTE: If filtering sediment-laden
water, a prefilter can be placed between the filter and the top retaining
screen.

d.

Dispense 10 to 20 mL of DIW from a wash bottle onto the filter.

e.

Close the plate-filter assembly by aligning the top and bottom
plates and lightly tightening the plastic bolts, followed by finger tightening
opposite pairs of bolts. Overtightening can cause the
plate-filter assembly to warp and leak.
Check that O-rings are in place before closing the assembly. Change
gloves.

2.

DH/CH: Pass the discharge end of the pump tubing through
the hole in the side or top of the processing chamber. Only the
CH team member touches sections of
tubing that will be in direct
contact with the plate-filter assembly.

•

Keep tubing as short as practical.

•

Attach a short piece of clean tubing to outlet connector of
plate-filter assembly.

3.

DH/CH: Rinse the plate-filter assembly with DIW, using
a peristaltic pump, as follows(rinsing must be repeated each time
a clogged filter is replaced with a new filter):

a.

CH/DH: Place intake end of peristaltic pump tubing into a 500-mL
container of DIW. Turn pump on low speed.

b.

CH: Open the air-vent valve on top of the plate-filter
assembly. Tilt the filter assembly slightly to the side and squeeze the
outlet tube closed to force trapped air out through the vent. Release the
outlet tube. (Venting trapped air is necessary because air bubbles will
reduce the effective filtering area by preventing sample from passing through
the filter.)

c.

CH: Close valve when top is filled with sample.

d.

CH: Pump sample through the plate-filter assembly and
discard this field-rinse water through the sink funnel or into the toss
bottle to prevent the water from ponding in the bottom of the processing
chamber.

e.

CH/DH: Remove intake end of the pump tubing from the
DIW container and continue to pump, draining as much of the remaining DIW
from the plate-filter assembly as possible.

4.

If using a peristaltic pump to transfer the sample to the
processing chamber (go to step 5 if sample delivery is with a submersible
ground-water pump):

a.

CH: Rinse intake end of the peristaltic pump tubing with the water
to be sampled

b.

CH: Transfer intake end of the peristaltic pump tubing
into the container of sample. If a churn splitter is used, transfer the
intake end through the churn funnel and reseal the plastic bag around the
tubing.

c.

CH: Remove peristaltic pump tubing from the inlet connector
of the plate-filter assembly and hold the end of the tubing over the sink
funnel or toss bottle.

d.

DH/CH: Start the peristaltic pump in the forward position
at slow speed and pump sufficient sample to fill and rinse all pump tubing.
Stop the pump after the tubing is rinsed.

5.

CH: Attach the discharge end of the peristaltic-pump
or submersible pump tubing to the inlet connector of the plate-filter assembly.

•

Keep tubing as short as practical.

•

A clean, small, plastic hose clamp can be used to secure the
discharge tubing to the inlet connector.

CH: Rinse appropriate sample bottles once with filtrate.
Filter no more than 100 mL of sample for the final rinse of all sample bottles
that require rinsing.

9.

Filter samples, filling bottles in the following order, as
applicable to study objectives and sample designation:

a.

Trace elements

TECHNICAL NOTE: Study objectives and
data-quality requirements govern procedures to be used if the filtered trace-element
sample is to reflect concentrations of analytes in true solution (the dissolved
fraction). Such interpretive studies of ground-water or surface-water chemistry
commonly use 0.1-mm
filter media and plate-filter assembly or a tangential flow method of phase
separation. Note that any deviation from the standard procedure for collecting
filtered trace-element samples through the 0.45-µm capsule filter
must be documented and reported with the analytical results.

b.

Major cations

c.

Nutrients, major anions, and alkalinity sample

d.

Radiochemicals

e.

Isotopes

10.

CH: If the filter medium clogs before the needed volume
of water is filtered, carefully remove the filter and replace with a new
filter. Repeat steps 1 through 7. Cap each bottle immediately after filling.

11.

If samples require chemical treatment
Go to
section 5.4.

12.

DH: After filtration,

a.

Check that information on the bottle label is complete and set the samples
aside for shipping (section 5.5). Samples that must be chilled need to
be refrigerated or packed in ice as quickly as possible and maintained
at 4°C without freezing.

b.

Disconnect and disassemble the plate-filter assembly. Discard
the used filter.

c.

Rinse all equipment with DIW immediately after use and before
it dries. Equipment that has dried after sampling without being rinsed or
cleaned needs to be cleaned vigorously with a detergent and rinsed with
DIW before the next use. Nonmetallic equipment must also be acid rinsed.

d.

Put rinsed tubing in a plastic bag for cleaning at the office
laboratory.

e.

If equipment is to be used at the next site, field clean all
the equipment using the procedures described in NFM 3. Field cleaning between
sampling sites is carried out while still at the sampling site.

13.

Document on field forms and in field notes any
modifications to the filtration procedures used.